mount-util: tweak how we find inaccessible device nodes

mount-util: tweak how we find inaccessible device nodes

On new kernels (>= 5.8) unprivileged users may create the 0:0 character
device node. Which is great, as we can use that as inaccessible device
nodes if we run unprivileged. Hence, change how we find the right
inaccessible device inodes: when the user asks for a block device node,
but we have none, try the char device node first. If that doesn't exist,
fall back to the socket node as before.

This means that:

1. in the best case we'll return a node if the right device node type
2. otherwise we hopefully at least can return a device node if one asked
   for even if the type doesn't match (i.e. we return char instead of
   the requested block device node)
3. in the worst case (old kernels…) we'll return a socket node